This invention relates generally to an emitter coupled multivibrator, and more particularly to a variable frequency multivibrator circuit configured so as to render the oscillation swing substantially independent of the charge/discharge current flowing through the circuit's timing capacitor.
Emitter-coupled multivibrator circuits are a subclass of constant-current charge/discharge oscillators and are known to have several performance advantages. First, since such circuits generally comprise all NPN components, they are capable of high-frequency operation (i.e. 10 megahertz). Second, these circuits are symmetrical producing symmetrical output waveforms which have low even-harmonic content provided the two halves of the circuit are matched. Third, substantially linear control of frequency may be obtained by means of a control current or voltage. A more detailed discussion of emitter-coupled multivibrator circuits can be found in Bipolar and MOS Analog Integrated Circuit Design by Allen B. Grebene (1984, John Wiley and Sons, Inc.).
In an elementary variable frequency emitter-coupled multivibrator, the output frequency F.sub.O may be thought of as EQU F.sub.O =I.sub.x /(2.multidot.C.multidot.V.sub.p)
where C is the capacitance of the timing capacitor, V.sub.p is the voltage across the capacitor, and I.sub.x is a control current. Thus, the frequency F.sub.O may be varied by varying I.sub.x ; however, a linear conversion of I.sub.x to F.sub.O would require that V.sub.p be kept constant.
One known approach for achieving a constant V.sub.p is through the use of clamping diodes. This circuit, however, is unsatisfactory since the output frequency is inversely proportional to V.sub.be (the base-emitter voltage drop of the transistor) resulting in a frequency having a strong positive temperature coefficient since V.sub.be varies at a rate of about -2 mV/degree.
A second approach to achieving a constant V.sub.p utilizes a current subtraction scheme to be more fully described herein below. In this approach, large resistors are used to provide good positive feedback; however, such resistors cause variations in V.sub.p. Furthermore, such resistors are temperature and process dependent which degrades the linearity of F.sub.O with respect to I.sub.x. Finally, the control current I.sub.x is related to the current in the feedback portion of the circuit which is in turn affected by variations in the resistors.